This invention relates generally to oil and gas well logging tools. More particularly, this invention relates tools for measuring rock formation properties such as density and porosity. This invention relates to an improved density tool using radiation detectors with improved operating characteristics at high temperatures that may be used in cased holes as well as open holes.
In petroleum and hydrocarbon production, it is desirable to know the porosity and density of the subterranean formation which contains the hydrocarbon reserves. Knowledge of porosity is essential in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of porosity is particularly useful in older oil wells where porosity information is either insufficient or nonexistent to determine the remaining in-place oil and to determine whether sufficient oil exists to justify applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas.
If the density of the formation is known, then porosity can be determined using known equations. A variety of tools exist which allow the density of the reservoir to be determined. Most of these tools are effective in determining the density (and hence porosity) of the reservoir when the wellbore in which the tool is run is an uncased reservoir and the tool is able to contact the subterranean medium itself. However, once a well has been cased, there exists a layer of steel and concrete between the interior of the wellbore where the tool is located and the formation itself. The well casing makes it difficult for signals to pass between the tool and the reservoir and vice-versa.
Many of the commonly used porosity and density measuring tools rely on the detection of gamma rays or neutrons resulting from activation of either a neutron source downhole or a gamma ray source. Fundamental to the detection of radiation is the use of scintillation counters for radiation detection. Scintillation is produced by ionizing radiation. The light flashes are typically converted into electric pulses by a photoelectric alloy of cesium and antimony, amplified about a million times by a photomultiplier tube, and finally counted. Scintillation counters permit high-speed counting of particles and measurement of the energy of incident radiation.
This arrangement has several disadvantages. Firstly, photomultiplier tubes require high voltages. The high voltage means that bulky insulation has to be provided. Photomultiplier tubes are inherently bulky, a disadvantage for downhole applications where space is at a premium. Their output can become noisy at the elevated temperatures encountered in boreholes. In order to reduce the effects of elevated temperatures, Dewar flasks may be used to keep the temperature down—another operational disadvantage. The noise becomes worse as the photomultiplier tube ages and has been exposed to long periods of vibration. Photomultiplier tubes can be damaged by vibration and the harsh conditions encountered downhole.
It would be desirable to have downhole radiation detectors that do not suffer from the drawbacks associated with photomultiplier tubes. The present invention satisfies this need.